Osmundaceae

Osmundaceae (royal fern family) is a family of ferns containing four to six extant genera and 18–25 known species. It is the only living family of the order Osmundales in the class Polypodiopsida (ferns) or in some classifications the only order in the class Osmundopsida. This is an ancient (known from the Upper Permian) and fairly isolated group that is often known as the "flowering ferns" because of the striking aspect of the ripe sporangia in Claytosmunda, Osmunda, Osmundastrum, and Plensium (subtribe Osmundinae). In these genera the sporangia are borne naked on non-laminar pinnules, while Todea and Leptopteris (subtribe Todinae) bear sporangia naked on laminar pinnules. Ferns in this family are larger than most other ferns. The stems of Osmundaceae contain vascular tissue arranged as an ectophloic siphonostele; that is, a ring of phloem occurs on the outside only of a ring of xylem, which surrounds pith (and no other vascular tissue). Stipules can be discerned at the leaf bases of these ferns. The hardened leaf bases are persistent and overlap to form a hardened layer surrounding the stem. The mantle of sclerenchymatous leaf bases and intermixed roots can form a woody trunk when the stem emerges above ground, up to in Todea barbara. Extinct members of the family, which flourished during the Mesozoic, could reach the stature of trees and be termed tree ferns. The leaves are either holodimorphic, with separate fertile and sterile fronds assuming an entirely different structure, or have fertile and sterile portions of the frond very distinct in structure. Sporangia in the Osmundaceae are large, and open at a slit on the top; the annulus that drives the sporangium opening is on the side. 128 to 512 spores are typically present. The spores are green, nearly round, and trilete. The spores germinate into gametophytes, which are green (photosynthetic) and grow at the surface. They are large and heart-shaped. The base chromosome number for members of the order is 22. Smith et al.